The present disclosure relates to an optical device, a method for manufacturing the same and a display device. In detail, the present disclosure relates to a hard coat layer-provided optical device.
In recent years, various display devices such as a liquid crystal display (LCD) and a plasma display panel (PDP) have become widespread. In a screen of such a display device, visibility especially in a bright place is remarkably inhibited by glare of external light such as sunlight and indoor illumination, and therefore, an optical film such as an anti-glare film capable of diffusing and reflecting external light on the screen surface is frequently used.
In this optical film, there has hitherto been employed a technique for configuring a fine concave-convex structure on the surface for the purpose of diffusing and reflecting the external light on the screen surface. Specifically, taking into consideration scratch resistance, a method for coating a diffusion layer having transparent fine particles dispersed in a hard coat coating material on a transparent plastic substrate is the mainstream of current liquid crystal display devices.
However, in the foregoing various display devices represented by recent thin-model flat-screen television sets, enhancement of image quality and high definition are rapidly advanced, and pixels become small-sized. For that reason, there are involved such problems that light transmitting through the optical film receives a strain by reflection or diffusion by the fine particles in the anti-glare layer or the surface concave-convex structure, whereby the image becomes unclear, or glare is generated as a glare phenomenon of brightness; and that the surface becomes discolored with respect to the image quality, whereby the definition is remarkably deteriorated. In consequence, the current optical films in which the surface concave-convex structure is formed using fine particles cannot sufficiently follow the foregoing enhancement of image quality and high definition, and hence, realization of an optical film in which the surface concave-convex structure is formed without using fine particles is being desired.
Meanwhile, in order to diffuse and reflect external light on the screen surface, as techniques for forming a fine concave-convex structure on the surface, there have hitherto been studied techniques for forming a fine concave-convex structure by embossing (shape transfer) as disclosed in JP-B-4-59605, Japanese Patent No. 3374299, JP-A-2004-29240 and JP-A-2005-156615.
JP-B-4-59605 proposes a method in which a fine concave-convex rough surface is formed on the surface of a polarizing plate protective film made of a cellulosic plastic by embossing, and a part of the surface layer of the fine concave-convex rough surface is further dissolved by an organic solvent to manufacture a non-reflective polarizing plate with high sharpness.
Japanese Patent No. 3374299 proposes a method for manufacturing an anti-glare film in which a rough concave-convex layer made of an ionizing radiation-curable resin and fine concaves and convexes along the surface thereof are formed on a transparent plastic film. According to this manufacturing method, rough concaves and convexes thereof are formed by any one of an embossing method, a sandblasting method or a resin convection method at the time of drying, and fine concaves and convexes thereof are made of a coating layer in a thin film form or formed by a lifting effect.
JP-A-2004-29240 proposes a method for manufacturing an anti-reflection film capable of imparting concaves and convexes onto the film surface by embossing. According to this manufacturing method, a plate to be used for embossing is regulated so as to have an arithmetical average roughness of concaves and convexes of 0.05 μm or more and not more than 2.00 μm and an average period of concaves and convexes of not more than 50 μm.
JP-A-2005-156615 proposes a method for manufacturing an anti-glare film in which in a fabrication process of a thermoplastic resin film, a film is stretched by a tenter before or after forming concaves and convexes on the surface thereof upon pressing a casting mold against the film surface, and a hard coat layer is formed on the resulting concave-convex surface.
As described previously, in JP-B-4-59605, a non-reflective polarizing plate with high sharpness is formed as a polarizing protective film by partially dissolving a fine concave-convex rough surface formed on the surface of a cellulosic plastic upon embossing in an organic solvent, thereby making a smooth concave-convex surface. However, this non-reflective polarizing plate is inferior in scratch resistance because a hard coat layer is not formed on its surface, and therefore, it is difficult to use this non-reflective polarizing plate for liquid crystal display products which are required to have durability, such as liquid crystal television sets. Also, JP-B-4-59605 does not describe the surface shape upon which anti-glare properties are revealed.
In Japanese Patent No. 3374299 and JP-A-2004-29240, though the surface shape is specified in terms of a surface roughness, the arithmetical average roughness statistically includes large and small complicated concaves and convexes. For that reason, the diffusion and reflection characteristics are not controlled at all, and the resulting anti-glare film becomes discolored, thereby producing a problem that the image quality is remarkably deteriorated.
JP-A-2005-156615 proposes an anti-glare film in which in a fabrication process of a thermoplastic resin film, a casting mold is pressed against the film surface to transfer a concave-convex shape thereonto, and a hard coat layer is coated and formed on the film surface. However, with respect to the surface shape, only a center line average roughness Ra on the concave-convex surface of the thermoplastic resin is limited to the range of from 0.05 to 10 μm (see, for example, claim 11), but the surface shape upon which anti-glare properties are revealed is not described at all. In consequence, the diffusion and reflection characteristics are not controlled at all, and the resulting anti-glare film becomes discolored, thereby producing a problem that the image quality is remarkably deteriorated.
As described previously, in the related-art optical films for revealing anti-glare properties due to the surface shape without using fine particles, the surface properties are specified in terms of a mere concave-convex shape; the surface shape is specified in terms of a surface roughness (arithmetical average roughness); or the surface shape is specified merely by diffusion and reflection characteristics (optical characteristics). However, a definite surface shape is not specified.
In consequence, it is desirable to provide an optical device from which high anti-glare properties and high contrast are obtainable without using fine particles, a method for manufacturing the same and a display device.